TT and earth cable sizes

[stl]

Hi,

I am looking to get a quote for someone to relocate a cooker control unit and install a cooker connection point, which seems fairly staightforward, but I have a query regarding the earth cables sizes in my flat.

The electric were installed in 1998 and have an earth cable (poss 6 or 10mm difficult to tell) from the CU to the adjacent gas meter. There is also a similar size cable going to the earth rod under the kitchen floor. This then goes from earth rod to all the combi boiler pipes and then to the bathroom where it connects metal pipes under the bath before going to the water stop cock supply pipes (under bath).

When I got a quote from British Gas they advised I needed to upgrade the earthing to 16mm to the earth rod and 10mm for all else and I gather this is as per the regs.

Question is - do I actually need to do this for safety and the change to the cooker unit control? and were cable sizes smaller for earths in 1998? - Also if I don;t have it done now, will it be a problem in the future if I ever sell flat or rent it out and it is picked up on a periodic inspection report.

Would be interested in knowing what are the real implications for having lower than standard c.s.a of the earth cable and main bonds?

Many thanks

 

[ezekiel]

the idea is for the resistance through the fault path and back to the supply to be very low. Then when a fault happens a very high current will flow rapidly blowing a fuse (or tripping a circuit breaker). That roughly the idea behind your main earth, equipotential bonding and circuit protective conductors.

The csa of cables can be calculated or more easily taken from the regulations.

sure someone can give you more info if needed

 

[Spark123]

A TT system is different to TN systems in the respect that often not enough fault current can flow in a L-E fault to cause the fuse/MCB etc to disconnect rapidly. This is why RCDs need to be used.
The Main Protective Bonding connects together the MET (main earthing terminal) and the extraneous conductive parts such as the water and gas supply. These need to be sized so they are not less than half that of the Earthing Conductor and not less than 6mm.
Is under the kitchen floor the best place for an electrode? I suppose as long as it is in damp soil then it will be OK - not entirely sure on this one.
The Earthing Conductor which connects the electrode to the MET is sized according to how it is installed. If it is burried then a sheathed copper conductor needs to be 16mm² unless it is mechanically protected by the likes of a metal conduit where as small as 2.5mm² may be used.
If it isn't burried then a 4mm² conductor can be used, or again 2.5mm² if mech protected.

 

[ericmark]

It is very easy when trying to give information as to permitted sizes to make mistakes I did it on this forum only a couple of days ago when I said needed 16mm and only needed 10mm.

The best way is to give reference numbers this means any one following can double check so 542.3.1 tells us where not protected against mechanical damage on buried we need 16mm² but from what you say it is not under ground. So 544.1.1 seems to be the one to look at and as far as I can see 6mm² is the lower limit where it is not TN-C-S or PME. Although Table 54.8 seems to say 10mm² and table 54.7 refers again to 16mm² so very confusing.

So moving away from regulations for a moment if the main earth is 6mm² it will take around 40 amp so the Ze would need to be 5.75Ω before an overload could take place and the earth rod where RCD is used has to be 200Ω looking at note 2 of table 41.5 and I know I needed 4 to 8 x 1.2m rods to get the 8Ω required when I was fitting earth rods so 6mm² would seem reasonable. One could argue that it should be linked to the Ze reading but so many times I has seen Ze entered as figures like 1.5Ω but on investigation I have found they did not disconnect the earth stake when measuring and 1.5Ω included the earths provided by water pipes and gas pipes and when re-measured gave reading like 30Ω so 6mm² was way over what is required.

So returning to question in real terms increasing the size of the main earth to above 6mm² will not do a thing.

Eric

 

[ericmark]

Oh it seems the forum does not know the ASC code for the ohm sign so shows Ω instead of ohms.

 

[Spark123]

Yep, we don't have an ohms sign. I tend to just type ohms, you can edit your posts by clicking the edit button at the top right hand side.

 

[securespark]

As Spark 123 says, if not mechanically protected, you can use 4mm² to connect an earth rod & 6mm² for the MET on TT supplies. Query this with BG.


IMO, it is not good practice to take the PEB to a rod. It should go back to the MET to which the rod is connected.

More importantly, do you have RCD protection? Have you got a stand-alone RCD? Have you got a metal clad consumer unit?

What is the Ze?

Post some piccys please!!

 

[Spark123]

It is very easy when trying to give information as to permitted sizes to make mistakes I did it on this forum only a couple of days ago when I said needed 16mm and only needed 10mm.

The best way is to give reference numbers this means any one following can double check so 542.3.1 tells us where not protected against mechanical damage on buried we need 16mm² but from what you say it is not under ground. So 544.1.1 seems to be the one to look at and as far as I can see 6mm² is the lower limit where it is not TN-C-S or PME. Although Table 54.8 seems to say 10mm² and table 54.7 refers again to 16mm² so very confusing.

544.1.1 is referring to main protective conductors. 54.8 is the sizing for PME, not for TT.
Using 54.7 to calculate the required size for the earthing conductor is one method of complying with 543.1.1, if you use the adiabatic with a TT system the sizes can be smaller. 543.1.1 is also where the minimum sizes of 4mm² when not buried and 2.5mm² if mech protected comes from.

 

[GaryMo]

Yep, we don't have an ohms sign. I tend to just type ohms, you can edit your posts by clicking the edit button at the top right hand side.

Ignore!

I placed an Ohms symbol in this message which displayed perfectly when previewing the thread but turned to rubbish when posted

 

[securespark]

My best advice is to ask the DNO if they can provide you with a PME connection & ditch the earth rod, in which case, 16/10mm² will apply.

 

[Taylortwocities]

When I got a quote from British Gas they advised I needed to upgrade the earthing to 16mm to the earth rod and 10mm for all else and I gather this is as per the regs.

Per the above, a TT sysytem, will be OK with a much smallaer csa than 16mm. Personally BG cant get my bills right so they've got no chance with anything difficult.

Your supplementary bonding to the bathroom should not go back to the earth terminal.

The earth electrode must be assessible. Under the kitchen floor is fine as long as there is a nice hatchway in the floor marked "EARTH ROD".

I agree with the suggestion that u get it converted to PME. Pics for me too please....

 

[stl]

much appreciated.

The entire CU is plastic case and protected by 30ma RCD. The earth cables are all laid loosely under the floor boards and not buried.

The main meter tails to the CU are 16mm and the earth cables to gas and rod are either 6 or 10mm but difficult to tell. From the earth rod, the 6/10mm cable goes to boiler pipework (metal parts before it goes into the all plastic system) and then by 4mm earth to metal bits of pipe under bath and then onto water stopcock under the bath..

Any further thoughts...? An electiain said would not need to update if doing other minor works and would just note on the certificate that cables are undersized. I am interested to know if this causes any safety risks or indedd could cause a problem if I rent out or sell flat?

Thanks

 

[Spark123]

The electrode should connect directly to the MET, not via other parts. The MET should then connect to extraneous parts such as your gas and the water supplies.
The 4mm in the bathroom should connect together all metallic pipes to the protective conductors of the circuits supplying any class 1 or 2 appliances in the location. This has slightly changed under the 17th edition of the regs and under certain circumstances, this supplementary bonding may be ommitted.